This invention relates to ester-azoxy compounds, and more particularly to ester-azoxy compounds having a nematic liquid crystal phase, which when added to a twisted nematic liquid crystal composition enlarge the liquid crystal temperature range, thereby improving liquid crystal display elements including the compositions.
It is generally known that nematic liquid crystal compounds are characteristic in that the electro-optical effect, such as light-scattering, double refraction and the like, changes greatly under the influence of an electric field. In view of these characteristics, nematic liquid crystals are used widely for liquid crystal display devices based on various display principles. The typical display principles include the twisted nematic display (TN-type) wherein the orientation of a nematic liquid crystal having a twisted configuration is controlled by application of an electric field across the crystal; the guest-host display (GH-type) wherein a guest-host effect is obtained by adding a dichroic dye to a TN-type display; and a phase transition display element wherein an optically active material is added to the liquid crystal and the spiral molecular arrangement is utilized.
The particular characteristics required of a liquid crystal material in order to be utilized in the above-mentioned display principles differ from each other. However, several conditions are required for all of them. First, the material must be stable chemically, and second it must be in the nematic state over a wide range of temperatures, encompassing room temperature.
Up to the present time, a single compound satisfying all of the above-mentioned conditions required for a liquid crystal display material has not been developed. Therefore, in order to obtain the desired properties in a liquid crystal material, various liquid crystal compositions which are mixtures of the various types of liquid crystal compounds have been utilized.
In order to provide a twisted nematic-type liquid crystal display (TN-LCD) having a large surface area, dynamic driving is required. Thus, liquid crystal compositions having improved dynamic characteristics are required. Particularly, the most significant property for improving dynamic driving is to provide a dynamic characteristic, for example the voltage-contrast characteristic. In other words, it is necessary to make the .beta. value closer to 1; .DELTA.=V.sub.90 /V.sub.10.
In the case of dynamic driving of a TN-LCD, the liquid crystal materials are severely limited in their dynamic characteristics, liquid crystal temperature range and stability. Conventional liquid crystal materials do not satisfy all of these limitations. For example, azoxybenzene derivatives while superior in dynamic characteristics to other liquid crystal materials suffer from shortcoming of light resistance. Most of the other materials which are utilized as a base material in a liquid crystal composition are not fully satisfactory in that the liquid temperature range is small. In order to avoid these problems, a material having a high clear point is added, but this causes a decrease in the dynamic characteristic of the material.
Accordingly, it is desirable to provide a compound which when added to the liquid crystal composition as a dopant or when used as a base of a liquid crystal material or dynamic driving of a TN-LCD will improve the dynamic characteristics thereof and increase the liquid crystal temperature range.